1. Field of the Invention
The present invention relates to a pellicle, a photomask, a pellicle frame, and a method for manufacturing a pellicle. More particularly, the present invention relates to a pellicle, which is used for protecting a reticle from attachment of a foreign matter. A reticle is used for a lithography process for manufacturing a semiconductor device such as an LSI (Large-Scale Integrated) circuit, a VLSI (Very Large-Scale Integrated) circuit, or an LCD (Liquid Crystal Display).
2. Description of the Related Art
To manufacture a semiconductor device such as an LSI circuit, a VLSI circuit, or an LCD by a lithography process, a light is irradiated to a semiconductor wafer through a reticle, on which a circuit pattern to be transcribed onto the wafer is formed. A reticle is a photomask used for a stepper. If foreign matter is attached to the reticle, this foreign matter absorbs or reflects the light, and thus the pattern transcribed onto the wafer may deform. Also, the shape of the edge of the pattern may deform. Therefore, the size, the quality, and the outward appearance of the pattern transcribed on the wafer deteriorate. Thus, the performance or the yield factor of a semiconductor device and LCD manufactured from this wafer decrease.
Because of the above-mentioned reason, the lithography process described-above is performed inside a clean room. However, even inside the clean room, it is difficult to always maintain the reticle in a normal condition, that is, the condition where the reticle is free from attachment of foreign matter. Thus, usually, a pellicle that protects the reticle from foreign matter is adhered to the surface of the reticle. A pellicle is transparent so that a light used for exposure can transmit through the pellicle onto the reticle.
By using a pellicle, the foreign matter does not attach to the surface of the reticle but attaches to the surface of the pellicle. Therefore, if the light is focused on the pattern of the reticle during the lithography process, the foreign matter, which attaches to the pellicle, does not influence the transcription of the pattern onto the wafer.
FIG. 1 shows a configuration of a conventional photomask. The photomask has a reticle 14 and a pellicle 100. The pellicle 100 includes a pellicle film 12 and a pellicle frame 10.
The pellicle 100 is manufactured by applying a good solvent of the pellicle film 12 on the top end face of a pellicle frame 10, drying the solvent on the pellicle frame 10 with wind, and adhering the pellicle frame 10 on the transparent pellicle film 12 as disclosed in Japanese Patent Application Laid-Open No. S58-219023. The pellicle frame 10 is made from material such as nitrocellulose, cellulose acetate, or fluoroplastics, which has high optical permeability. The pellicle frame 10 can be made from material such as aluminum, stainless steel, or polyethylene, as examples.
The pellicle 100 can also be manufactured by adhering the pellicle frame 10 on a pellicle film 12 with adhesive 20 as disclosed in U.S. Pat. No. 4,861,402, Japanese Patent Application publication No. S63-27707, and Japanese Patent Application Laid-Open No. H7-168345. Acrylic resin, epoxy resin, or fluoroplastics can be used as adhesive 20. Also, an adhesive layer 22 and a separation layer for protecting the adhesive layer 22 is further adhered on a bottom end face of the pellicle frame 10. An adhesive layer is made from material such as polybutene resin, polyvinyl acetate resin, acrylic resin, or silicon resin. Therefore, the pellicle film 12 is adhered to the pellicle frame 10 via the adhesive 20. Then, the photomask 200 is manufactured by adhering the pellicle 100 on the reticle 14 with the adhesive 22.
Recently, the requirement for the resolution of the lithography has gradually increased. Therefore, a light having a shorter wavelength is gradually used to satisfy the requirement of the resolution. Specifically, instead of an ultraviolet light of g-line having 436 nm wavelength or i-line having 365 nm wavelengths, an extreme ultraviolet light of KrF excimer laser has been used recently. In the near future, a vacuum ultraviolet light such as an ArF excimer laser having 193 nm wavelength or an F2 excimer laser having 158 nm wavelengths will be used.
If the wavelength of the light becomes shorter, the photon energy of light becomes large. Thus, the deterioration of the material used for the lithography process becomes large. Especially, in a case of the pellicle film 12, because the material used for the pellicle film 12 is organic material such as fluoroplastics, the pellicle film 12 tends to deteriorate easily when using the light having a shorter wavelength.
Therefore, it is proposed to use an inorganic material for a material of a pellicle film 12. In a case of using an inorganic material for the pellicle film 12, the pellicle film 12 should have a larger thickness than the pellicle film 12 made of organic material in order to have enough strength because the inorganic compound layer is generally weak. However, if the thickness of the pellicle film 12, which is made of inorganic material, is large, it becomes very important that the surface of the pellicle film 12 and the surface of the reticle 14 are accurately parallel to each other because of the optical restriction inside the stepper.
Therefore, comparing the relationship between the surface of the pellicle film 12 made of organic material and the surface of the reticle 14, the surface of the pellicle film 12 made of inorganic material and the surface of the reticle 14 should be more accurately parallel.
If the pellicle film 12 is adhered to the pellicle frame 10 through the adhesive 20 as shown in FIG. 1, it is difficult to make the surface of the pellicle film 12 be accurately parallel to the surface of the reticle 14 because the thickness of the adhesive 20 is not uniform.
Generally, the pellicle 100 is adhered to the reticle 14 through the adhesive 22, which is applied on the bottom end of pellicle frame 10. By pressing and adhering the bottom end face of the pellicle frame 10 to the reticle 14, the adhesive layer protected by the separation layer of the adhesive 22 is collapsed, and the pellicle 100 thus be adhered to the reticle 14.
Because this adhesive 22 has elasticity, the distance between the bottom surface of the pellicle film 12 and the top surface of the reticle 22 changes according to the pressure applied on the pellicle 100 against the reticle 14. If the distribution of weight applied on the pellicle 100 is different in each location inside the surface of the pellicle 100, to which the reticle 14 is adhered, the height of the adhesive 22 becomes different for each location. Therefore, the surface of the pellicle film 12 and the surface of the reticle 14 do not become parallel to each other.